The invention pertains to a method of making a cemented carbide cutting tool, as well as the cemented carbide cutting tool itself.
There are cemented carbide (e.g., tungsten carbide-based materials with a cobalt binder) cutting inserts that exhibit a surface zone of non-stratified binder enrichment such as disclosed in U.S. Pat. No. 4,610,931 (and U.S. Reissue Pat. No. 34,180) to Nemeth et al. and U.S. Pat. No. 5,955,186 to Grab.
U.S. Pat. No. 4,548,786 to Yohe discloses a process for making a cemented carbide cutting insert with surface binder enrichment wherein a dewaxed blank that does not contain nitrogen is exposed during the heating process to an atmosphere with a nitrogen partial pressure. PCT Patent Publication No. 98/16665 to Lindskog et al. discloses a cemented carbide cutting insert with surface binder enrichment which uses a nitrogen atmosphere for a part of the process. European Patent No. 0 569 696 to Uchino et al. pertains to a cemented carbide cutting insert that contains zirconium and/or hafnium and has a zone of surface binder enrichment underneath the cutting edge. European Patent No. 0 603 143 to Gustafson et al. discloses a method for producing a coated cemented carbide with a zone of stratified binder enrichment that includes sintering a compacted body containing nitrogen in an inert atmosphere (or a vacuum) followed by a cooling at a specific rate.
Kennametal KC850 grade coated cutting insert (KC850 is a registered trademark of Kennametal Inc. of Latrobe, Pa., USA, for cutting inserts) has a zone of stratified binder enrichment. The Nemeth et al. article entitled xe2x80x9cThe Microstructural Features and Cutting Performance of the High Edge Strength Kennametal Grade KC850xe2x80x9d, Proceedings of Tenth Plansee Seminar, Reutte, Tyrol, Austria, Metalwerke Plansee A.G. (1981), pages 613-627 describes the Kennametal KC850 grade cutting insert. The article by Kobori et al. entitled xe2x80x9cBinder Enriched Mayer Formed Near the Surface of Cemented Carbidexe2x80x9d, Funtai oyobi Funtai Yakin, Vol. 34, No. 3, pages 129-132 (1987) describes stratified binder enrichment.
Other articles discuss the occurrence of a zone of binder enrichment in cemented carbides. These articles include Schwarzkopf et al., xe2x80x9cKinetics of Compositional Modification of (W,Ti)Cxe2x80x94WCxe2x80x94Co Alloy Surfacesxe2x80x9d, Materials Science and Engineering, A105/106 (1988) pages 225-231, Gustafson et al., xe2x80x9cBinder-Phase Enrichment by Dissolution of Cubic Carbidesxe2x80x9d, Int. J. of Refractory Metals and Hard Materials, 12 (1993-1994), pages 129-136, Suzuki et al., xe2x80x9cThe B-Free Layer Formed Near the Surface of Sintered WCxe2x80x94Bxe2x80x94Co Alloy Containing Nitrogenxe2x80x9d, Nippon Kinzoku Gakkaishi, Vol. 45, No. 1 (1981), pages 95-99, and Suzuki et al., xe2x80x9cThe B-Free Layer Formed Near the Surface of Vacuum-Sintered WCxe2x80x94Bxe2x80x94Co Alloys Containing Nitrogenxe2x80x9d, Transactions of the Japan Institute of Metals, Vol. 22, No. 11 (1981), pages 758-764.
While some of the above articles, patents and products disclose or comprise cutting inserts that exhibit adequate performance, there remains a need to develop processes that produce products (and the products themselves) that have better properties. In this regard, it would be desirable to provide a process (and the resultant product) that sinters the blank in an atmosphere most always having at least a partial pressure so as to be able to control the depth of the zone of binder enrichment. Such a process would provide for an optimum balance between the edge strength and the deformation resistance of the substrate. Such a process would also provide for excellent consistency in the depth of the zone of binder enrichment for the parts throughout a heat.
It would also be desirable to provide a process, as well as the resultant product, wherein there is no carbon precipitation in the zone of binder enrichment, especially in a substrate that has a core porosity of greater than COO according to ASTM Designation B276-91 (Reapproved 1996). The absence of such carbon precipitation would enhance the adhesion of the coating to the substrate.
It would be advantageous to provide an as-sintered cemented carbide that exhibits a surface zone of non-stratified binder enrichment (or essentially non-stratified binder enrichment which means that most of the binder enrichment is of the non-stratified type with a slight (or small) amount of stratified binder enrichment) wherein there is enhanced solid-solution hardening. In this regard, a cemented (cobalt) tungsten carbide substrate that has nitrogen atoms present at the interstices of the cobalt atoms facilitates solid-solution hardening. The enhancement of solid-solution hardening is especially true for a substrate that with a bulk region that exhibits a porosity of greater than C00 according to ASTM Designation B276-91 (Reapproved 1996). In such a case, the atomic radius of nitrogen (about 0.75 Angstroms)is smaller than the atomic radius of carbon (about 0.91 Angstroms).
It would be advantageous for applying a coating, and especially a coating that contains nitrogen (e.g., titanium nitride or titanium carbonitride), directly on the surface of a substrate that contains nitrogen. In the case of the application of a coating of titanium nitride on the surface of a substrate that has bulk region with a porosity of not greater than C00 according to ASTM Designation B276-91 (Reapproved 1996), the presence of nitrogen would promote nucleation of titanium nitride. In the case of the application of titanium carbonitride to the surface of a substrate with a bulk region exhibiting a porosity of greater than C00 according to ASTM Designation B276-91 (Reapproved 1996), the presence of carbon and nitrogen would help promote the nucleation of titanium carbonitride.
It is believed that with the presence of additional nitrogen in the cobalt binder for a cemented (cobalt) tungsten carbide substrate that has a surface zone of cobalt enrichment, there is an increase in the chemical affinity between the substrate and a nitrogen-containing coating, such as, for example, titanium nitride or titanium carbonitride. It is believed that such an increase in the chemical affinity should lead to an increase in the adhesion of the coating to the substrate.
It is believed that an increase in the availability of nitrogen in the cobalt near the surface of the substrate should reduce the potential for the formation of a brittle eta phase at the interface between the coating and the substrate. The reduction in the potential to form eta phase permits the use of substrates that have lower carbon contents.
It is believed that a higher nitrogen content in the substrate should also result in a decrease in the grains size of the tungsten carbide. An increase in the N/(C+N) content should lead to a decrease in the grain size of the tungsten carbide. The tungsten carbide phase content in the microstructure should increase to a maximum as the N/(C+N) ratio increases.
It can thus be seen that there is a belief that it would be advantageous to provide an as-sintered cemented (cobalt) tungsten carbide substrate that has a higher nitrogen content. The higher nitrogen content should increase adhesion strength between the coating (especially a coating such as titanium nitride and titanium carbonitride) and the substrate. The higher nitrogen content in the cobalt binder near the surface of the substrate should reduce the potential for the formation of brittle eta phase at the coating-substrate interface. The higher nitrogen content should decrease the grain size of the tungsten carbide.
Typically, it has been necessary to use different compositions of the starting powder to produce either an as-sintered substrate that exhibits a surface zone of binder enrichment or an as-sintered substrate in which there is an absence of a surface zone of binder enrichment. As can be appreciated, there is an increase in the cost associated with storing (and/or making) two or more different compositions of starting powder as compared with the cost of storing (and/or making) only one composition of starting powder. From a production viewpoint, it would advantageous to provide a process that would utilize a single starting powder composition to selectively produce either an as-sintered substrate of a commercial quality with a surface zone of binder enrichment or an as-sintered substrate of a commercial quality that does not have a surface zone of binder enrichment.
In one form, the invention is a coated cutting insert that includes a tungsten carbide-based substrate with rake and flank surfaces and a cutting edge at their intersection. The substrate, which has a porosity rating according to ASTM Designation B276-91 (Reapproved 1996) of greater than C00, has a surface zone of non-stratified binder enrichment that does not exhibit any carbon precipitation. There is a coating on at least a part of the substrate.
In another form thereof, the invention is a method of making a coated tungsten carbide-based cutting insert wherein starting powders are mixed, pressed into a green blank which is then dewaxed. The dewaxed blank is subjected to a sinter heating step, a sinter holding step and a controlled cooling step wherein all of these steps occur in their entirety in an atmosphere that has a partial pressure and for at least a part of the duration of the sinter heating step and the sinter holding step the atmosphere contains a nitrogen partial pressure. The as-sintered substrate is then coated with one or more layers.
In still another form thereof, the invention is a cemented (cobalt) tungsten carbide-based substrate made by sintering a mass of compacted powders in an atmosphere that contains at least a partial pressure. The substrate has rake and flanks surfaces that have a cutting edge at their intersection. The substrate has a zone of non-stratified cobalt enrichment that is adjacent to and extends inwardly from the cutting edge and at least one of the rake and flank surfaces toward the bulk substrate, which has a porosity of greater than C00. The zone of cobalt enrichment does not exhibit any carbon precipitation and has a maximum cobalt content between about 125 and about 300 percent of the bulk cobalt content.
In yet another form thereof, the invention is a made by sintering a compacted mass of starting powders in an atmosphere having at least a partial pressure wherein the starting powders containing the following components: cobalt, tungsten, carbon, titanium, niobium and tantalum, the substrate comprising: a peripheral surface defined by a rake surface, a flank surface, and a cutting edge at the intersection of the rake and flank surfaces; the substrate having a zone of non-stratified cobalt enrichment beginning adjacent to and extending inwardly from the cutting edge and at least one of the rake surface and the flank surface toward a bulk region, the bulk region having a porosity according to ASTM Designation B276-91 (Reapproved 1996) of greater than C00; the zone of cobalt enrichment being at least partially depleted of the solid solution carbides and/or solid solution carbonitrides; the zone of cobalt enrichment not exhibiting any carbon precipitation; and the zone of cobalt enrichment having a cobalt content between about 125 percent and about 300 percent of the cobalt content of the bulk region.
In still another form thereof, the invention is a coated cutting insert that comprises a substantially fully dense substrate made by sintering a compacted mass of starting powders in an atmosphere containing a nitrogen partial pressure. The starting powders include the following components: a binder selected from one or more of cobalt, nickel, iron and their alloys wherein the binder is present between about 3 weight percent and about 12 weight percent, up to about 95 weight percent tungsten, up to about 7 weight percent carbon, and up to about 13 weight percent of one or more of the following components: titanium, tantalum, niobium, hafnium, zirconium, and vanadium. The substrate has a rake surface and a flank surface, and there is a cutting edge being at the intersection of the rake and flank surfaces. The substrate has a zone of non-stratified binder enrichment of a generally uniform depth beginning adjacent to and extending inwardly from the cutting edge and at least one of the rake surface and the flank surface toward a bulk region. The zone of binder enrichment has a high nitrogen content, and the bulk region of the substrate has a high nitrogen content. There is a coating on the cutting edge and at least a portion of one or both of the rake surface and the flank surface of the substrate.
In another form thereof, the invention is a method of making a coated cemented carbide cutting insert comprising the steps of: blending starting powders to form a starting powder mixture wherein the powders contain the following components: a binder selected from one or more of cobalt, nickel, iron and their alloys, tungsten, carbon, and one or more of the following: titanium, tantalum, niobium, hafnium, zirconium, and vanadium; pressing the starting powder mixture to form a green cutting insert blank; dewaxing the green cutting insert blank to form a dewaxed cutting insert blank; sinter heating the dewaxed cutting insert blank from about the maximum dewaxing temperature to at least a pore closure temperature in an atmosphere having a first nitrogen partial pressure for substantially the entire sinter heating step so as to form a pre-sintered cutting insert blank; sinter holding the pre-sintered cutting insert blank at a sinter hold temperature in an atmosphere having a second nitrogen partial pressure for substantially the entire sinter holding step to form a sintered cutting insert blank wherein the second nitrogen partial pressure is greater than the first nitrogen partial pressure; cooling the sintered cutting insert blank from the sintering temperature to a target temperature below the eutectic temperature so as to form an as-sintered cutting insert substrate having a peripheral surface with a zone of non-stratified binder enrichment beginning adjacent to and extending inwardly toward a bulk region of the substrate; and coating the as-sintered cutting insert substrate with a coating comprising one or more layers including a base layer on the surface of the substrate, and the base layer comprising a material containing nitrogen.
In still yet another form thereof, the invention is a method of selectively making either as as-sintered substrate that exhibits a surface zone of binder enrichment or an as-sintered substrate that does not exhibit a surface zone of binder enrichment, the method comprising the steps of: blending starting powders with an effective amount of nitrogen being absent and containing a binder alloy selected from one or more of cobalt, nickel, iron and their alloys, tungsten, carbon, and one or more of the following: titanium, tantalum, niobium, hafnium, zirconium, and vanadium; pressing the starting powder mixture to form a green cutting insert blank; dewaxing the green cutting insert blank to form a dewaxed cutting insert blank; sinter heating the dewaxed cutting insert blank from the maximum dewaxing temperature to at least a pore closure temperature in an atmosphere having a first nitrogen partial pressure for substantially all of the entire sinter heating step so as to form a pre-sintered cutting insert blank; sinter holding the pre-sintered cutting insert blank at a sinter hold temperature in an atmosphere having a second nitrogen partial pressure for substantially the entire sinter holding step as to form a sintered cutting insert blank and wherein the second nitrogen partial pressure may selectively be either greater than equal to or less than the first nitrogen partial pressure; cooling the sintered cutting insert blank from the sintering temperature to a target temperature below the eutectic temperature so as to form an as-sintered cutting insert substrate wherein when the second nitrogen partial pressure is greater then the first nitrogen partial pressure the as-sintered cutting insert substrate does not exhibit a surface zone of binder enrichment and when second nitrogen partial pressure is equal to or less than the first nitrogen partial pressure the as-sintered cutting insert substrate exhibits a surface zone of binder enrichment; and coating the as-sintered cutting insert substrate with a coating comprising one or more layers.